The use of biomass power plants for energy generation is positive in terms of CO2
emissions, but is often connected with high levels of other pollutants because of for
example incomplete combustion. For smaller grate fired boilers combusting high moisture
fuel, high emissions of CO are often a challenge. Because of the environmental
impact of CO and the additional species associated with the detection of CO, the emission
limits are restrictive and are expected to be tightened further in the future. Concerning
new plants, the emission limits are often in the range of 150 to 200 mg/Nm3
at 6% O2. Much research has been directed to improving the combustion, mostly focussed
on mixing and air staging. Appropriate control of the furnace temperature or
injection of species could possibly be useful in reducing the CO emissions. This is the
concern of this report.
Numerical models of the detailed chemical kinetics in a combustion process was used
to study this in a basic manner, through simulations in Cantera. This was based
on a representative high moisture fuel. The kinetics were described by a chemical
mechanism, GRI-1.2, with 177 reactions. Studying the affect of different temperatures
through simulations it was found that an appropriate temperature interval for CO
burnout was 1300 K to 1800 K. Through investigation of the effect on the kinetics by
adding different species, it was found that injection of steam could possibly increase
the burnout rate of CO by increasing the amount of OH radicals in the combustion gas.
Also, the effect of injecting steam with secondary air is of practical interest, as moist
air, for example from a drying process, is used in some plants.
Both subjects were investigated further through CFD simulations of a simplified biomass
furnace. The detailed chemical kinetics were succesfully incorporated into the CFD
simulations using the Eddy Dissipation Concept (EDC) model. Turbulence was modelled
using the k-ǫ approach, and simulations were carried out in FLUENT. Four different
cases were simulated, a reference case, a case with water injection and two cases
with different parts of the furnace walls insulated.